1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive comprising depletion mode metal oxide semiconductor field effect transistors (MOSFETs) for protecting a head from electrostatic discharge (ESD).
2. Description of the Prior Art
FIG. 1 shows a prior art disk drive comprising a disk 2, an actuator arm 4 rotated about a pivot, and a head 6 attached to a distal end of the actuator arm 4. The disk 2 and head 6 (together with other electronics, such as a preamp circuit not shown) form a channel. The head 6 comprises a first head terminal 8A and a second head terminal 8B extending from a read element. Electrostatic discharge (ESD) can damage the head 6 while inactive, such as during manufacturing, while the disk drive is powered down, or while the head 6 is not being used to read the surface of the disk 2 during normal operation of the disk drive. Magnetoresistive (MR) heads which employ a highly sensitive MR read element are particularly susceptible to ESD, and presumably highly sensitive future head technology will be susceptible to ESD. The prior art disk drive of FIG. 1 employs an ESD protection circuit 10 comprising an array of six diodes for protecting the head 6 while inactive. A first pair of diodes 12A limit the voltage across the head terminals 8A and 8B, a second pair of diodes 12B provide a conductive path for the first head terminal 8A to ground 14, and a third pair of diodes 12C provide a conductive path for the second head terminal 8B to ground 14.
The diodes 12A–12C of the prior art ESD protection circuit 10 provide limited protection since they have a turn on voltage of 0.7v which may be too high for certain heads. That is, an ESD voltage of less than 0.7v may damage the head 6 before the diodes 12A–12C turn on. In addition, coupling six diodes 12A–12C to the head terminals 8A and 8B increases the capacitance which decreases the bandwidth (data rate) of the channel.
There is, therefore, a need for a disk drive with improved low voltage ESD head protection as well as reduced channel capacitance to enable faster data rates.